Development in light-emitting diode (LED) lighting technology continues to take place at a dramatic pace. LED chip makers have raised the luminous efficacy of LED components. One of the technology used inside the LED device is the distributed Bragg reflector (DBR). DBR increases the overall amount of light exiting the LED device.
In its simplest form, a distributed Bragg reflector is a quarter wave stack of dielectric materials. The materials from which the layers of the stack are made alternate from layer to layer down the stack. The materials are selected such that the alternating layers have a high index of refraction, then a low index of refraction, then a high index of refraction, and so forth down the stack. The distributed Bragg reflector is typically designed to maximize the reflection of a particular wavelength of light. The upper layer of the stack has a thickness of one quarter of the wavelength of light at the design wavelength of the distributed Bragg reflector. This wavelength is the wavelength of the light when the light is passing through the layer. The wavelength λ, frequency f, and speed of light (velocity v) is given by the equation λ=v/f. When light leaves one medium and enters another medium, the speed and wavelength of the light may change but the frequency does not change. The material from which the upper layer is made therefore determines the speed of light v in the medium. The material also influences the wavelength λ of the light in the upper layer.
Each material has an index of refraction η. The index of refraction η is the ratio of the speed of light in a vacuum to the speed of light in the medium. The wavelength of light in a medium is given by the equation λ=λo/η, where λo is the wavelength in a vacuum. Light traveling through air is traveling at close to the speed of light in a vacuum, so the wavelength of light in air is close to wavelength of the light in a vacuum. The quarter wavelength of light through the medium of a layer is determined using the relationship QWOT=λo/4η, where η is the refractive index of the material from which the layer is made. In this way, the refractive indices of the materials of the various layers of the stack are used to determine how thick to make each layer of the stack so that the layer has a thickness of one quarter wavelength. The design wavelength λo for the distributed Bragg reflector is typically designed to be longer than the wavelength of the light emitted by the LED. For example, the optimal DBR design wavelength is around 510 nm for an LED that emits mostly blue light at 450 nm.
Light passes into the stack and through the upper layer, and then some of the light reflects off the interface between the upper layer and the next layer down in the stack. Part of the light proceeds down into the next layer of the stack to the next interface. If the interface is one from a low-index medium to a high-index medium, then any light reflected from the interface will be phase shifted by 180-degree. If, on the other hand, the interface is one from a high-index medium to a low-index medium, then any reflected light will have no phase shift. Each interface causes a partial reflection of the light wave passing into the stack. The phase shifts, in combination with the thicknesses of the layers of the stack, are such that the portions of light reflecting off interfaces all return to the upper surface of the stack in phase with each other. The multiple reflections off the interfaces all combine at the top of the stack with constructive interference. The result is that the Distributed Bragg Reflector has a high reflectivity within a finite spectral range known as the stop-band.
Though DBR increases the optical reflectivity, it is currently limited to the LED devices only. The LED packaging, however, does not take advantage of the DBR. In LED packaging, the LED device is directly placed on top of a metal substrate, coated with highly reflective metal layer. The reflective metal layer in a LED package is not optimized to reflect lights emitted from the LED devices. Further, the reflective metal layer is prone to environment degradation over time because the reflective layer is not protected.
A design for an LED packaging is sought that improves the reflectivity of the reflective layer and to protect the reflective layer from fast degradation.